1. Field of the Invention
The present invention relates to a servo controller for controlling a servomotor for driving a feed shaft in a machine tool or a manufacturing machine, an arm of an industrial robot, etc. and in particular to a servo controller for a servomotor for a gravitating axis to which gravitation is exerted in the machine having a mechanical brake for applying a braking force on the gravitating axis.
2. Description of Related Art
A movable element in a machine such as a feed shaft of a machine tool or an manufacturing machine and an arm of an industrial robot is driven by a servomotor and a position of an axis connected to the movable element is controlled by the servomotor to be shifted to a commanded position and maintain the commanded position. When the servomotor is turned from an operational state to a non-operational state, i.e., an excitation of the servomotor is discontinued due to an emergency stop, an outage of the power source, etc., the position of the axis to which an external force is exerted is displaced. Particularly, with respect to an axis to which the gravitation is exerted in an operating direction thereof, which is referred to as a gravitating axis, the gravitating axis always receives the gravitation and the position of the gravitating axis displaces downward by the gravitation when the excitation of the servomotor for driving the gravitating axis is discontinued.
Conventionally, when an emergency stop is commanded in an operation of the servomotor for driving the gravitating axis, a velocity command in the servo system for the servomotor is made xe2x80x9c0xe2x80x9d to stop the motion of the gravitating axis as soon as possible. Then, a command to operate the mechanical brake is issued and after a time period (typically 100 ms-200 ms) sufficient for the mechanical brake to apply the braking force on the axis, the excitation of the servomotor is discontinued. With the above sequential control, the down displacement of the gravitating axis is prevented in the emergency stop.
When the excitation of the servomotor is discontinued in the state of applying the braking force on the gravitating axis by the mechanical brake, the gravitational force on the gravitating axis, which has been borne by the output torque of the servomotor, is borne solely by the mechanical brake. A frictional contact part of the mechanical brake is made fixed to the gravitating axis and the gravitational force is applied to the contact part. As a result, the position of the gravitating axis is displaced downwardly by an amount of a backlash formed by a torsion between a proximal portion and a distal contacting portion of the contacting part. It has been difficult to prevent the downward displacement of the gravitating axis caused by the backlash of the mechanical brake.
An object of the present invention is to provide a servo controller capable of preventing a downward displacement of a gravitating axis on which gravitation is exerted when an excitation of a servomotor for driving the gravitating axis is discontinued.
A servo controller of the present invention comprises: brake operation commanding means for issuing a brake operating command to a mechanical brake for applying a braking force on a gravitating axis to which gravitation is exerted when an excitation of a servomotor for driving the gravitating axis is discontinued; and offset commanding means for issuing an offset command for driving the servomotor such that a position of the gravitating axis is shifted by an offset amount in a direction opposing to the gravitation exerted on the gravitating axis when the excitation of the servomotor is discontinued.
The offset amount may be set not less than a backlash in the mechanical brake, so that the downward displacement of the gravitating axis due to the backlash of the mechanical brake is within the offset amount.
In the case where the servo controller performs a position loop control of the servomotor, the offset command may be provided to the position loop control as a position command.
In the case where the servo controller performs a velocity loop control, the offset command may be provided to the velocity loop control as a velocity command to drive the servomotor at a predetermined velocity for a predetermined time period.
The excitation of the servomotor may be discontinued when an emergency stop is commanded, when an outage of a power source of the servo controller is detected and when the machine does not operate for a set time period.
It is preferable that a shifting motion of the gravitating axis due to the offset command is completed before the braking force of the mechanical brake is actually applied on the gravitating axis.
The offset command and the brake operating command may be issued simultaneously when the excitation of the servomotor is discontinued.